Anchors for headers with HTML

Using aglio `2.0.4` with olio `1.2.1`, the following

``` markdown
# Group Important Stuff

## Bar

```

leads to the following HTML

``` html

Important Stuff
¶

Bar

¶

```

Though the generated anchors are straight forward, the following seems more natural to me:

``` html

Important Stuff
¶

Bar

¶

```

so that you can refer to `Important Stuff` by `#important-stuff` and to `Bar` by `#header-bar`

In my opinion HTML tags and their attributes should be ignored when generating anchors.

Reproduce the issue with the provided aglio 2.0.4 and olio 1.2.1 versions using the two Markdown headings, then locate the anchor-generation entry point. Done means generated IDs and permalink hrefs strip HTML tags and attributes, producing `important-stuff` and `header-bar`.

Written by the indexing model from the issue text.